1. Field of the Invention
This present invention relates to a process for preparing titanium and .alpha.and (.alpha.+.beta.) titanium alloy materials comprising a fine acicular microstructure and having a superior fracture toughness and fatigue properties.
2. Description of the Related Art
Titanium and titanium alloys are used in various of material applications, such as aerospace and structural components for automobiles, due to their high strength-to-density ratio and excellent corrosion resistance, and the applications thereof are increasing. The properties required of these materials in general are a good fracture toughness and high fatigue strength, and a structural material satisfying the above-described requirements must have a metallographically fine microstructure.
Titanium and titanium alloys are supplied in the form of plates, wires, rods, tubes or shapes and generally manufactured through a combination of hot working with heat treatment, but in the prior art processes, it is difficult to prepare a product having a homogeneously fine microstructure. Specifically, with respect to commercial pure titanium, since the impurity contents are limited, it is difficult to homogeneously refine the microstructure. On the other hand, the .alpha. and (.alpha.+.beta.) titanium alloys have a drawback in that a proper working temperature range is too narrow to satisfy, during the hot working, both a requirement of a good workability for obtaining a very precise product shape and a requirement for forming a fine microstructure.
Examples of known processes for preparing the above-described alloys include that disclosed in Japanese Unexamined Patent Publication No. 58-100663, wherein a primary working is conducted in a .beta. region having a good workability and a finish working is then conducted in an (.alpha.+.beta.) region, and that disclosed in Japanese Patent Publication No. 63-4914, wherein the heating and working are repeated in a narrow temperature range in an (.alpha.+.beta.) region, to thereby form a fine equiaxed grain microstructure.
In these processes, however, a high order working must be conducted in an (.alpha.+.beta.) region wherein the hot workability is poor, and thus the productivity is very poor due to the occurrence of hot tear cracking, etc. Further, the resultant microstructure is not sufficiently refined. For this reason, in some cases, as specified in AMS4935E, the finish working is conducted in a .beta. region wherein the working can be easily conducted. In this case also, since the working is conducted in a .beta. region at ahigh temperature, not only does the .beta. grain per se grow to a large size, but also it is difficult to prepare a desired fine acicular microstructure even when quenching is subsequently conducted.
Specifically, in titanium and existing .alpha. and (.alpha.+.beta.) titanium alloys, since the .beta. transformation point is high (for example, about 885.degree. C. for JIS grade 2 titanium, about 1040.degree. C. for .alpha. Ti-5Al-2.5Sn, and about 990.degree. C. for (.alpha.+.beta.) Ti-6Al-4V), the .beta. phase per se is coarsened. Further, since the Ms point is high (for example, about 850.degree. C. for JIS grade 2 titanium, about 950.degree. C. for .alpha. Ti-5Al-2.5Sn, and about 850.degree. C. for (.alpha.+.beta.) Ti-Ti-6Al-4V) an acicular martensitic phase is decomposed into an (.alpha.+.beta.) phase during cooling from the .beta. region temperature. Therefore, the material prepared according to the conventional process comprises a mixed structure composed of a coarse lamellar .alpha. phase formed from a coarsened .beta. phase, and a residual .beta. phase. This material is disadvantageously inferior to a material having a fine microstructure, from the viewpoint of such properties as the fatigue strength, etc., thereof.
Further, the above-described poor hardenability unfavorably renders the structure heterogeneous, due to the difference in the hardenability of the surface layer and of the central portion of the material, depending upon the size of the material.
If the lowering in the .beta. transformation point or Ms point is intended to solve the above-described problems, the addition of substitutional alloying elements, such as V, Cr and Mo, to the titanium and .alpha. and (.alpha.+.beta.) titanium alloys suffices for this purpose. The addition of the above-described elements, however, causes the composition of the material to become different from that intended, which renders this method unusable.
As apparent from the foregoing description, to date, a conventional process has not been found effective for the forming of a microstructure which is easy to work, and for converting the resultant microstructure into a fine acicular microstructure.